The present invention relates to fuel cells. In particular, the present invention relates to the maintenance of fuel cell cathode air quality.
Fuel cells provide an increasingly popular way to generate electricity. Fuel cells are characterized by a favorable power density and a high specific power output. In addition, fuel cells generally produce benign waste products. As a result of the benefits of fuel cells, they are being considered for and applied to an expanding number of applications requiring electrical power.
Fuel cells can be beneficially employed in connection with a wide variety of devices requiring a portable electrical power supply. For example, fuel cells have been used in connection with electric automobiles. The fuel cell is desirable in such applications because they are capable of providing a high specific power output in a relatively lightweight and compact package. In addition, fuel cells that produce harmless waste products are available. For example, the byproduct of the reaction between hydrogen and oxygen, the two components of a typical fuel cell, is water. Fuel cells are also useful in connection with providing a reliable power supply for various other devices that might be used in remote locations and/or in connection with portable devices. For example, fuel cells can be used as a power source for radios and other communication devices, global positioning system receivers, portable computer systems, night vision equipment and other devices. As can be appreciated, many such devices can be beneficially employed in man-portable systems. For example, such systems may be used by outdoor enthusiasts, soldiers, surveyors, or other persons requiring a reliable and easily portable source of electrical power.
In a typical fuel cell, hydrogen molecules (H2) are converted to electrons (exe2x88x92) and protons (H+) in a platinum catalyst, thus forming the anode of the fuel cell. The protons flow through a proton exchange membrane (PEM). At the cathode of the fuel cell, the protons are combined with oxygen molecules (O2) and electrons to form water (H2O). Electrical devices may be provided with electrical power by interconnecting them between the cathode and the anode of the fuel cell.
In a fuel cell such as the one described above, the proton exchange membrane may be a polymer electrolyte membrane. The membrane may be constructed from a perfluorinated polymer based material (e.g. NAFION, available from E.I. du Pont de Nemours and Company), that is hydrophilic and that contains sulphonic acid groups that form negatively charged transfer sites, allowing the membrane to conduct positively charged ions. Accordingly, the positively charged hydrogen ions are capable of passing through the membrane to react with oxygen molecules and electrons at the cathode to form water. In this way, the natural tendency of the protons to oxidize and form water is used to produce electricity that can be applied to the performance of useful work. The chemical equations for the above-described processes are as follows:                                           2            ⁢                          H              2                                ⁢                      →            _                    ⁢                      xe2x80x83                    ⁢                                    4              ⁢                              H                +                                      +                          4              ⁢                              e                -                                                                          (                      at            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            anode                    )                                                                        4              ⁢                              e                -                                      +                          4              ⁢                              H                +                                      +                          O              2                                ⁢                      →            _                    ⁢                      xe2x80x83                    ⁢                      2            ⁢                          H              2                        ⁢            O                                                (                      at            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            cathode                    )                                                                            2              ⁢                              H                2                                      +                          O              2                                ⁢                      →            _                    ⁢                      xe2x80x83                    ⁢                      2            ⁢                          H              2                        ⁢            O                                                (                      overall            ⁢                          xe2x80x83                        ⁢            reaction                    )                    
In a single fuel cell, the above-described reaction produces an electrical potential of about 0.7 Volts. Therefore, in a typical application, fuel cells are combined into a fuel cell stack to produce a desired voltage. Furthermore, by increasing the surface area of the individual fuel cells, the current producing capacity of the fuel cell may be increased.
One of the advantages of fuel cells is that they are capable of utilizing oxygen in the ambient atmosphere as one of the components of the reaction used to generate electrical power. However, the proton exchange membrane of the fuel ceil is vulnerable to fouling by contaminants. In particular, the negatively charged transfer sites in the proton exchange membrane can become irreversibly occupied by metal ions (e.g. Na+, Mg2+, Fe2+, Cr+, Ni2+, etc.), preventing protons from reacting with oxygen at the cathode. Such ions may be introduced to the interior of the fuel cell in a liquid solution, or as airborne salts. Therefore, measures must be taken to exclude liquids and salts from the fuel cell. This need is particularly acute when use of the fuel cell in marine environments is contemplated.
Conventional methods for excluding chemical contaminants from the fuel cell cathode air stream have included direct particle filtration in combination with a consumable chemical getter material. The getter material may include an adsorption material such as a zeolite or activated charcoal or other substance that reacts chemically with the contaminants to trap them and thereby filter them from the air stream. However, such methods are incapable of preventing liquid water from being aspirated into the air stream and fouling the getter material. If the getter material becomes overwhelmed by the amount of water that has been aspirated, the water may come in contact with the proton exchange membrane and ions in the water may foul the proton exchange membrane.
Conventional systems for preventing liquids from being aspirated into a fuel cell and fouling the proton exchange membrane have used valves or other active measures. However, such active measures are unreliable, and typically admit at least some water into the fuel cell, hastening the need to replenish the getter material or allowing at least partial fouling of the proton exchange membrane. Accordingly, conventional methods do not provide a satisfactory method or apparatus for protecting fuel cells from fouling by contaminants carried by liquids, particularly where the fuel cell will likely be exposed to water or airborne salts.
For the reasons set forth above, there is a need for a method and apparatus for maintaining fuel cell cathode air quality. In particular, there is a need for a method and apparatus capable of maintaining fuel cell cathode air stream quality reliably, and without a need for complicated and expensive active components. Furthermore, there is a need for such a method and apparatus that are capable of excluding liquids and airborne salts from a cathode air stream without requiring the use of consumable components. Additionally, the method and apparatus should be reliable in operation and inexpensive to implement.
In accordance with the present invention, a method and an apparatus for maintaining the quality of a fuel cell cathode air stream are provided. In particular, the method and apparatus of the present invention allow liquids and airborne salts to be excluded from a cathode air stream without the use of active components. The method and apparatus of the present invention generally provide for the filtering of a cathode air stream through a filter formed from a gas permeable, hydrophobic, microporous, polymer film. The use of such a filter allows for the reliable operation of a fuel cell even in an environment in which airborne salts are prevalent or in which immersion of the fuel cell is likely, such as in marine environments.
According to one embodiment of the present invention, a substantially waterproof enclosure is provided for housing a fuel cell. A filter having a gas permeable, hydrophobic, microporous, polymer membrane is provided in a wall of the enclosure, to allow air to be admitted into the enclosure. The pore size of the membrane is selected to provide a desired water intrusion pressure, while admitting a required flow of air for a given filter surface area.
According to another embodiment of the present invention, a pump is housed within the enclosure. Depending on the demand on the fuel cell for electrical current, the pump draws air through the filter and provides that air to the fuel cell cathode. According to a further embodiment of the present invention, exhaust from the fuel cell is allowed to exit the enclosure through a check valve.
According to one embodiment of the present invention, a method for maintaining the air quality of a cathode air stream is provided. According to the method, a cathode air stream is drawn through a gas permeable, hydrophobic, microporous, polymer film, after which it is provided to the fuel cell cathode. According to another embodiment of the present invention, the method for maintaining the quality of a cathode air stream includes sizing the pores of the filter membrane to exclude particles of greater than a selected diameter, and to exclude liquid water at up to a selected pressure.
Additional advantages of the present invention will become readily apparent from the following discussion, particularly when taken together with the accompanying drawings.